1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a compound semiconductor layer of a group III-V nitride-based semiconductor (hereinafter referred to as a nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride) or TlN (thallium nitride) or mixed crystals thereof.
2. Description of the Prior Art
In recent years, a nitride-based semiconductor laser device emitting blue or violet light is actively subjected to research and development as a recording or reproducing light source employed for an optical disk system having a high density and a high capacity.
FIG. 8 is a typical sectional view showing the structure of a conventional nitride-based semiconductor laser device. The semiconductor laser device shown in FIG. 8 is formed by successively stacking a buffer layer 2 of undoped AlGaN, an undoped GaN layer 3, a first contact layer 4 of n-GaN, a crack preventing layer 5 of n-InGaN, a first cladding layer 6 of n-AlGaN, an emission layer 7 of an InGaN multilayer film, a second cladding layer 8 of p-AlGaN and a second contact layer 9 of p-GaN on a C plane of a sapphire substrate 1 by MOCVD (metal-organic chemical vapor deposition).
Partial regions between the upper surface of the second contact layer 9 and a prescribed depth of the second cladding layer 8 are removed for forming a ridge portion 10. A partial region between the second cladding layer 8 and a prescribed depth of the first contact layer 4 is removed for forming an electrode forming surface 11.
A p-side electrode 131 is formed on the upper surface of the second contact layer 9. An n-side electrode 132 is formed on the electrode forming surface 11 of the first contact layer 4.
A dielectric film 12 of SiO2 or SiN is provided for limiting a region fed with a current and obtaining optical density sufficient for lasing. The dielectric film 12 is formed on both side surfaces of the ridge portion 10, flat surface portions of the second cladding layer 8, the side surfaces of the layers from the second cladding layer 8 to the first contact layer 4 and the upper surface of the first contact layer 4.
The dielectric film 12 also has a function of preventing electrical shorting across the p-side electrode 131 and the n-side electrode 132.
When prepared from SiO2, the dielectric film 12 is disadvantageously cracked or removed due to insufficient adhesion between the nitride-based semiconductor and the SiO2 film. Therefore, the yield is reduced.
When the dielectric film 12 is prepared from SiN, it is difficult to form an SiN film having a thickness of several 100 nm with uniform quality since the content of N (nitrogen) strongly depends on film forming conditions. Therefore, reproducibility of device characteristics is reduced because of variation of an etching rate for forming an opening with the N content of the SiN film
While influence by such dispersion of the etching rate can be reduced by reducing the thickness of the SiN film, it is difficult to completely cover a step portion in this case. If the thickness of the SiN film is reduced below 30 nm, presence of pinholes results in a reduction of voltage resistance and/or a leakage current.